OK,I have a 6' (cantilever)extension on the deck with the 2X8's hanging out another 12"(with a tapered corner on the base for decorative),I want to extend the deck out about 12' further.

Knowing the weight will be much greater with the new 12' extension I will need to support the joint where I plan on sistering to the existing joists(5 sets of 2X8's-4'O/C.)

I would prefer not to have posts going straight up for each joint(I maintain the ground below).

Option 1) Install 4X4 posts at angles from base of wall to joint.

Option 2) Install 1 4X4 (vertical)at the center of the extension and branch out each direction(knee bracing),but I think this would only suffice for the middle 3 sets,not the outer 2.

Option 3) Install 2 4X4 (vertical)on the 2nd and 4th sets of joists,bracing to the sides for full support.

Option 4) Posts to all sets of joists:(, not much of an option,mowing and maintaining around 1,let alone 5 would be a nightmare.

Outside the 12' extension will be posts attached to an existing foundation wall(from original barn),there I can do all 5 posts,no problem,it's inside the maintained area I want a different solution.

Thanks,

Ken

Hi KBC I keep re reading your post and Im trying to wrap my head around this. Correct me if Im wrong.

Help me convert this to Job lingo. What's "Each Joint" a 'Joist?

Here's where I get Lost. The 5 sets 2x8 4' O.C.The word "sets" are confusing me. All my brain thinks is (A Joist, A joist band, A post, A cantilever, A sister joist, etc,

So lets think of this as building a Bridge over a river ( Grass below) One point is supported at the barn area, other support at existing cantilever support point. Clear span will be approx 18 feet. If we went for cemetery an angle brace at both sides will suffice to stop the deflection and cut down clear span.

I'm assuming you have 8' below this bridge area. Reason I ask is the lower that is in height under the New deck extension iit cuts down the distance an angle brace can reach for more load bearing capacities.

Think of it this way. You now have 6' cantilever. Add 12 and its all divisible by (6) well we know the first 6 foot there works. So if we generically angled braced off the end of that cantilever to its back support point were now supporting the existing cantilever. Coming from the other direction we have another angle brace 6 foot away from the barn. So past the point is 3' ( Half of the 6" clear span )

Bottom line here is an angle brace Will work depending on what beef you have in the original cantilever support point. That point is the deflection issue for angle bracing.

So when finished, side view is 18 feet of deck with full supports on both ends. From those points are angle braces that go out approx 6 towards the middle, which leaves unsupported middle total clear span at 6'

Each joint=where I will have to sister joists together.

Sets= a 2X8 on each side of a 4X4 post(this deck was run with the barn's interior floor joists which were doubled 2X's making the deck joists(original)a 3 1/2" spread allowing for a 4X4 post for railings.

OK,,now for the support.

At each joist sistering,I have 2-2X8's spread by a 4X4,in order to have support under the joists(joint)at the point of sistering,I am now thinking of adding 2-2X8's inside the existing 2X8's(they extend out 14"),notching the new ones to allow the angled 4X4 to have a stop.ONLY go 8-10' in the middle with these 2X8's,then sistering to them to make the 3 1/2" opening for the posts on the outer edge.Make sense?(2 outside a 4X4,2 inside to span ,then 2 outside them at 6' from the end to allow the 4X4)

Yes,this is a very open,highly visible area and it will eventually be a tourist/visitor center so it needs to not only have curb appeal,but perhaps nice barn style joinery and strength.It will be an extension of an observation platform.

KBC If the public is going to use this deck system an engineer here would be the best ticket to cover the Butt.

That said I was thinking of your situation and what popped into my head was a trestle conf or part of one. Where you want to cut the 2x8 for a "Stop", My thought was this. If we looked at the angel braces and the deck above it would be part of an (A) frame basic construction. So if you added an extra board a foot or so under the deck and used it as a collar Beam this would be the "Stop" for that top angel board. Now if you took that a step further continue that board to each end of the deck posts.

Then you would get Positive lock and bracing on the angel braces and also it would look a bit like a covered trestle bridge.

Unfortunately I LIVE with an engineer:p(retired many years and full of piss and vinegar :D )

I understand the idea of a collar beam, not the image of the continued board to the deck posts, maybe it's too early to think:rolleyes:

Quote:

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Originally Posted by KBC

Unfortunately I LIVE with an engineer:p(retired many years and full of piss and vinegar :D ) I understand the idea of a collar beam,,not the image of the continued board to the deck posts,,maybe it's too early to think:rolleyes:

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That's funny... My gals son is an engineer, part of his learning was on my job sites. My Job sites knocked some piss and vinegar of him.

Top of the A is your deck elevation point. The sides of the A are the new angel braces but where they meet the bottom of the deck bridge there is 6' clear span.The middle line of the A is the collar beam but it continues past each side of the angled part of the A. then stops at the uprights that support the deck.